1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, which is provided with a catalyst arranged in the exhaust system of the engine, and air-fuel ratio sensors arranged in the exhaust system at locations upstream and downstream of the catalyst, for detecting the concentration of a specific component in exhaust gases emitted from the engine, and which controls the air-fuel ratio of an air-fuel mixture supplied to the engine, in response to outputs from the air-fuel ratio sensors.
2. Prior Art
An air-fuel ratio control system for internal combustion engines is already known, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 5-106493, which includes air-fuel ratio sensors arranged in the exhaust system of the engine at locations upstream and downstream of a three-way catalyst arranged in the exhaust system of the engine, respectively, and controls the air-fuel ratio of an air-fuel mixture supplied to the engine in a feedback manner responsive to outputs from the air-fuel ratio sensors. The air-fuel ratio control system according to the above publication also has a function of detecting deterioration of the three-way catalyst based on the output from the air-fuel ratio sensor downstream of the three-way catalyst.
In some areas in the world (e.g. in the East of the United States of America), a gasoline containing a large amount of sulfur (e.g. 300 to 800 ppm) is used as fuel for engines. If a gasoline with such a high sulfur content is used in an internal combustion engine, the catalyst arranged in the exhaust system of the engine is poisoned by sulfur present in the gasoline and its oxygen storage capacity is degraded.
However, the conventional catalyst deterioration-detecting method does not contemplate the sulfur poisoning of the catalyst. As a result, when the catalyst undergoes sulfur poisoning, the catalyst is determined to be deteriorated even though it is a new one. Thus, the conventional method cannot discriminate the sulfur poisoning of the catalyst from the deterioration of the catalyst due to aging.